Analog amplifiers such as operational amplifiers (op amps) have been widely produced using a variety of technologies, such as Bipolar semiconductors. More recently, complementary metal-oxide-semiconductor (CMOS) technology has been employed for such analog amplifiers. See for example U.S. Pat. No. 5,670,910 by Kato, and assigned to NEC Corp. Sometimes such analog amplifiers are integrated onto the same silicon substrate as digital sub-systems, in a mixed-signal integrated circuit (IC).
FIG. 1 is a high-level diagram of an amplifier connected in a unity-gain configuration. Amplifier 10 has differential inputs + and -, known as non-inverting and inverting inputs. The difference in the voltages of signals applied to the + and - inputs is amplified and output from amplifier 10 as signal Vo. Amplifier 10 is known as a differential amplifier, since the difference in input voltages is amplified, rather than the absolute voltage on any one input.
Input voltage Vin is applied to the non-inverting + input of amplifier 10. Both input voltage Vin and output voltage Vo are referenced to a ground, normally 0 volts. The output voltage Vo is fed back to the inverting - input of amplifier 10. Since amplifier 10 amplifies any difference in voltage between its two inputs, amplifier 10 is in a stable steady-state condition when the two input have the same voltage. Amplifier 10 adjusts output voltage Vo until it matches the input voltage Vin. When Vo is lower than Vin, amplifier 10 sees a positive voltage difference on its inputs and drives the output voltage Vo higher until Vo reaches Vin. When Vo is above Vin, amplifier 10 sees a negative voltage difference on its inputs and drives the output voltage Vo lower until Vo reaches Vin.
Since the feedback to amplifier 10 acts to drive its output Vo to the same voltage as its input Vin, the voltage amplification or gain in steady state is one. The feedback connection of amplifier 10 is known as a unity-gain configuration. Note that the current delivered by amplifier 10 may increase if Vin increases to reduce output sink current; thus a large current gain can still be provided by the unity-gain amplifier. Such a unity-gain amplifier is sometimes known as a voltage-follower circuit. Such unity-gain amplifiers are often used as a buffer to increase drive capacity.
FIG. 2 is a circuit for a CMOS analog amplifier that implements the unity-gain amplifier of FIG. 1. The drain of p-channel transistor 26 supplied current to resistor 28, forming a voltage-reference generator. The IR drop through resistor 28 determines the bias voltage that is applied to the gates of p-channel transistors 26, 22, 20, so that p-channel transistors 22, 20 act as current sources with currents referenced to the current through p-channel transistor 26.
P-channel transistors 12, 14 form a differential pair that switch current from p-channel transistor 22 to either n-channel transistor 16 or 18. A current mirror is set up by n-channel transistor 16, 18, since their gates are connected together, providing the same gate-to-source voltage Vgs. Together, p-channel transistors 22, 12, 14 and n-channel transistors 16, 18 form a CMOS differential amplifier.
The drain of transistor 12 is connected to the gate of n-channel output transistor 24. An output stage is composed of p-channel transistor 20 and n-channel transistor 24. However, since p-channel transistor 20 has its gate driven by the bias voltage from resistor 28, p-channel transistor 20 acts as a current source, outputting a constant current. Only the sink current through n-channel output transistor 24 varies.
The output voltage Vo is taken from the drains of transistors 20, 24, and fed back to the gate of p-channel transistor 14. Thus one of the differential pair of transistors 12, 14 is driven by Vin while the other is driven by Vo. This provides the unity-gain configuration with feedback of Vo.
When Vin rises above Vo, p-channel transistor 12 turns off more than transistor 14, so more current passes through transistors 14, 18. The gate-to-source voltage of transistor 18 must rise to accommodate the higher current flow. This higher gate voltage is mirrored to n-channel transistor 16, resulting in more current through n-channel transistor 16. Since more current is passing through n-channel transistor 16, but less current through p-channel transistor 12, the drains of transistors 12, 16 fall in voltage. This voltage drop is applied to the gate of output transistor 24, reducing the current sink through transistor 24. The reduced current sink, together with a constant current source from p-channel transistor 20, raises the output voltage Vo until Vo matches the rise in Vin.
While such an analog amplifier is useful, the amount of current from the output stage is limited. The output current is equal to the current sourced from p-channel transistor 20, minus the current sinked through n-channel output transistor 24. Since p-channel transistor 20 has a fixed gate voltage, the source current is constant. Only the sink current changes as the gate of n-channel output transistor 24 is varied. In practical cases, the maximum source current to the output is about half of the constant current which p-channel transistor 20 can supply. The gate voltages of transistor 24 must stay within a limited range for the circuit to operate properly. This limits the variation of sink current that can occur.
Much of the current through output transistors 20, 24 is wasted, being sent from power to ground, with a limited amount of current being sourced to the output. A large output current is often required by loading of the output, forcing high currents through transistors 20, 24. This high current is undesirable from a power-budget view, since the chip's power-supply current specification must increase to supply the current to p-channel transistor 24. Heat dissipation can also be a concern, and large transistor sizes may be needed that require more die area, increasing cost.
What is desired is a unity-gain amplifier constructed from CMOS transistors. A lower-power amplifier output stage is desired that can still source and sink a large current from the output. Reduced power-to-ground current in the output stage is desired. An active push-pull output stage for a unity-gain amplifier is desired. Reduced waste current in a high-current-drive amplifier is desired.